The term "cross-fading" is used in the art to describe the superposition of two scenes respectively fading in and out. Thus, with the use of a suitable dissolving shutter as described for example in U.S. Pat. No. 3,635,549, the fadeout of a preceding scene is followed by reverse transportation of the film with the shutter closed whereupon part of the film is re-exposed with a fade-in effect leading into the subsequent scene. As further described in that prior patent, a takeup reel inside the film cassette is driven at less than normal speed in the first phase of such a trick shot--i.e. during the fadeout -- whereby a reserve length of film accumulates in the cassette for subsequent withdrawal in a second phase, i.e. during the reverse run.
The accumulation of a reserve length of film by the stepping or slowing of a windup operation is not necessary if the camera is equipped with means for selectively releasing the usual rewind stop of a film cassette. Such releasable rewind stops are often provided in sound-film cassettes but are usually lacking in cassettes of the silent-film type. A camera designed to handle both types of cassettes is therefore sometimes provided with a sensor designed to detect the presence of a sound-film cassette as distinct from a silent-film cassette which is generally smaller than the former, the sensor being used to deactivate the stop-release mechanism if there is no sound-film cassette in the camera; see commonly owned U.S. Pat. No. 4,065,209.
Only a limited length of excess film can be accumulated in a cassette, in the vicinity of its takeup reel, for subsequent withdrawal if the reel does not have a releasable rewind stop. Such accumulation may be sufficient for cross-fading purposes but would be unsuitable for other types of trick cinematography in which it is desired to run extended scenes or perhaps the entire film in reverse to create an effect of backward motion upon reproduction. The same applies to situations in which a major part of a film is to be exposed twice, as for background scenery and foreground action, without cross-fading.